The invention relates to the field of semiconductor devices and optical inspection systems. More, particularly, the present invention relates to surface defect detection of semiconductor devices using optical imaging.
Solar cell panels have long been used in spacecraft for critical power requirements. The development of a high speed optical method for inspecting solar cells is responsive to a recognized need for a transportable visual inspection method that inspects for solar cell damage in large arrays both at the fabrication facility and after shipment at the launch site. The potential for damage during transport to the launch site and growth of existing cracks beyond acceptable limits remains an important issue, particularly for the more vulnerable, newer generation cells based on thin germanium substrates. Concern over potentially damaged cells has grown with the introduction of the newer thin germanium substrate cells. Typically, individual solar cells are examined during array fabrication using close-up visual techniques that require considerable training and inspection time under dedicated conditions such as close access, appropriate lighting, and optical magnification. Current inspection methods rely on trained inspectors, using aided or unaided visual techniques to examine specimens for cracks, flaws or other irregularities under optimum conditions of lighting, accessibility, and time. An inspector would typically find flaws while searching under low magnification with a mobile light source. Current methods for documenting flaws and cracks generally rely upon hand drawn cell maps or multiple still photographs to document the observed flaw features requiring extensive operator time and training, and is subject to variable human interpretations.
Such inspections insure that critically damaged cells are replaced, and that minor defects are within acceptable parameters prior to launch. Due to constraints of time and access, similar comprehensive inspections of very large arrays after shipment to the launch site are not easily performed. The primary constraints at the launch site are restricted access to the cell surface in that a standoff distance is maintained, and limited time that is available for conventional cell inspection. Hence, there is need for an examination technique that can facilitate the inspection of large arrays at the fabrication site and at the launch site in order to assess possible shipment damage. Currently, there are no automated, image-enhancing, or systematic recording devices to assist and document the solar cell inspection process. Non-standard illumination and detection, such as infrared light and infrared viewing camera has been used to enhance crack detection for high efficiency silicon solar cells that exhibit very low visible light reflectance. This approach alone, however, would not satisfy the concerns for a remote, high rate, inspection process.
U.S. Pat. No. 5,894,345, issued Apr. 13, 1999 to Takamoto entitled xe2x80x9cOptical Method of Detecting Defects and Apparatus Used Thereinxe2x80x9d, discloses a linear array line-type source of dotted lines that does not flood a specular surface with a contrasting pattern from a conventional light source. The system detects light emitted from the surface as scattered light as in conventional illumination methods.
U.S. Pat. No. 5,359,416, issued Oct. 25, 1999 to Mueller, entitled xe2x80x9cSystem and Process for Detecting and Monitoring Surface Defectsxe2x80x9d, discloses an inspection system for viewing defects at an angle other than the angle of the expected reflectance, and hence does not take advantage of the reflective specular surface of the device that may contain cracks and flaws.
U.S. Pat. No. 5,091,963, issued Feb. 25, 1992 to Litt, entitled xe2x80x9cMethod and Apparatus for Inspecting Surfaces for Contrast Variationsxe2x80x9d, discloses the use of contrast variations in reflected light produced by the surface of an article, but does not apply the contrast variations to inspect specular surfaces.
U.S. Pat. No. 5,047,851, issued Sep. 10, 1991 to Sauerrwein, entitled xe2x80x9cProcess and Device for Detecting and Evaluation Surface Cracks in Workpiecesxe2x80x9d discloses how conventional inspection images can be digitized and computer processed in a way to distinguish surface cracks, but does not use contrasting illumination patterns to enhance the detectability of small flaws.
U.S. Pat. No. 5,351,078, issued Sep. 27, 1994 to Lemelson, entitled xe2x80x9cApparatus and Method for Automated Observation of Objectsxe2x80x9d discloses aspects of automating a conventional inspection process where light is focused on the image, but not the reflecting surface.
These cited patents do not address the special difficulty in examining specular surfaces, but are primarily directed to regular, non-mirror-like scattering surfaces that do not take advantage of traversing contrasting reflected patterns that reveal very small and subtle defects and flaws. These and other disadvantages are solved or reduced using the invention.
An object of the invention is to provide a method for inspecting specular surfaces for flaws and cracks using a contrast illumination pattern.
Another object of the invention is to provide a method for inspecting specular surfaces using a contrast pattern and viewing video camera that concurrently traverse the specular surface for detecting differences in the reflection for locating defects on the specular surfaces.
The present invention relates to the visual inspection of smooth, specular surfaces, that is, mirror-like surfaces having high reflectivity. The inspection method is based on the design and application of contrasting patterned images to enhance the detection of cracks or other physical disruptions such as pits, dents, bumps, particulate contamination, among others, on the surface of specular materials, or specular surfaces that are covered by a transparent material, such as solar cells under coverslips.
The inspection method uses the specular mirror-like surface to image a contrast pattern or lighting mask. The human eye, or the video camera, views the contrasting patterned image reflected by the mirror-like surface focused on the plane of the specular surface. Small cracks or flaws are enhanced because the slight change in the tilt of the surface around the flaw causes the out-of-focus image of the contrast pattern to appear distorted. In order to inspect a specular surface area, the image of the contrast pattern scans or translates across the inspection surface. The flaws are revealed over time as the contrast pattern traverses across the specular surface. Hence, the reflected image usually does not contain a complete flaw or crack in any one instant of time. Contrast variations on a mirror-like inspected specular surface are illuminated by a pattern or mask having a contrast pattern. The contrasting pattern floods the inspection specular surface with conventional illumination for illuminating and viewing the mirror-like specular surface to detect small flaws.
Thus, the camera or human eye and the contrasting pattern light source move relative to the inspection surface, while maintaining a fixed angle of incidence. The inspection method enhances the viewing of defects by gathering reflected light, preferably with a video camera, at an angle that is at an angle of expected reflectance. The camera mechanically traverses an inspection range to maintain the reflection of the pattern in view while scanning. The inspection system detects optically nonhomogeneous portions of the specular surface of the device under test for flaws or cracks by the absence of light reflected at the expected angle of reflectance
The inspection method allows a video camera to capture detailed surface imagery of individual devices, such as cells in a large array, employing an illumination contrast pattern during the scan that enhances the image of the non-planar surface around a crack or flaw. With an arrangement of a camera, an illuminator, and a contrasting pattern, the scanning method offers a reproducible, systematic examination process that is independent of the inspection operator or local ambient conditions. A videotaped inspection provides a real image record of flawed conditions. The method offers assured compatibility with launch site environments and helps to facilitate eventual comparison of pre-shipment and post-shipment array conditions to determine whether pre-existing cracks grow beyond acceptable limits, and to assess the extent of new crack formation.
The visible inspection system acquires and stores detailed imagery of the cell and coverslip condition with sufficient spatial resolution and image quality to verify whether small pre-existing cracks have grown beyond acceptable limits, and to determine the extent of new crack formations. The system is amenable to launch site constraints by providing remote or standoff inspection capability, with an image collection time fast enough to make data acquisition at the launch site for subsequent detailed evaluation practical for very large arrays. That is, the system provides a real time continuous video record of the inspection process. The system provides the means to perform the inspection task in a practical time period, to rapidly obtain inspection data even when close access is prohibited and when imaging of flaws is necessary. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.